The present invention relates to an image forming apparatus such as an electrophotography copy machine or an electrostatic recording apparatus, and more particularly, to an image forming apparatus for recording a toner image transferred onto a recorded sheet by heating and fixing the toner image.
Hitherto, there are copy machines for fixing a toner image to a recorded sheet by using a fixing roller which is heated. In the copy machines, a continuous copying operation causes the quantity of heat taken by the recorded sheet to be increased in proportion to the number of passing recorded sheets. Thus, a surface temperature of the fixing roller is gradually decreased because of the shortage of heat supplied by a heater in the fixing roller. FIG. 8 is a diagram showing change, with time, in a surface temperature of the fixing roller in the conventional copying machine. Referring to FIG. 8, a curve e shows change, with time, in the surface temperature of a sheet passage unit in the fixing roller, and a curve f shows change in a sheet non-passage unit with time. Herein, the sheet passage unit corresponds to a portion of the fixing roller which comes into contact with the recorded sheet and the sheet non-passage unit corresponds to a portion of the fixing roller which does not come into contact with the recorded sheet. During a period T0 of time in FIG. 8, a standby mode of the copying machine is shown. As shown in FIG. 8, at a time tx, staring the continuous copying operation, with passing time, that is, with the increase in number of copied sheets, the surface temperature of the sheet passage unit of the fixing roller is reduced. When the surface temperature of the sheet passage unit is reduced to a prescribed temperature or less, toner on the recorded sheet cannot be melted by heat, thereby making it impossible to fixing the toner. To prevent the failure of fixing the toner, power to be applied to a fixing heater may be increased. However, by using the method, power consumption over the copying machine is increased and, then, the copying machine is unavailable for a home power supply. Consequently, there is an inconvenience in that a set place of the copying machine is limited.
Then, conventionally, the countermeasures are taken against the inconvenience as follows. In other words, a temperature detecting element detects a surface temperature of the fixing roller, and when the surface temperature of the fixing roller is reduced to be less than the prescribed value, a copying process is intermitted and the apparatus is in the standby mode until the surface temperature of the fixing roller is returned to be a predetermined temperature.
However, according to the conventional method, when the surface temperature of the fixing roller is reduced to a prescribed value (lower limit value) or less during the continuous copying operation, the copying operation is interrupted halfway of the continuous copying operation and an operator waits for a state in which the surface temperature of the fixing roller recovers to the predetermined temperature. As a consequence, the conventional method has a problem to take a long time for the continuous copying operation.
Also, in the conventional copying machine, upon continuous copying operation, it is difficult to keep a distribution of the temperature of the fixing roller in the longitudinal direction uniform. This is remarkable when only a single heater for heating the fixing roller and only a single temperature detecting element for detecting the surface temperature of the fixing roller can be provided in terms of costs. That is, in a fixing device having a plurality of light-emitting-type heaters, the change in surface temperature of the fixing roller in the longitudinal direction can be reduced as much as possible by finely controlling light-on timings of the plurality of heaters having different light distributions. On the contrary, in the case of using only the single heater, obviously, the light distribution is fixed and, therefore, the distribution of the temperature of the fixing roller in the longitudinal direction cannot be keep uniform only by the on/off control. The temperature detecting element detects only a temperature nearby the portion against which the temperature detecting element abuts. Therefore, the temperature nearby the portion in which the temperature detecting element is provided is controlled so as to be the predetermined temperature and, however, the temperature excluding the above portion becomes too much higher or too much lower. For example, in the case of continuous passage of post cards, as a recorded sheet, having a width much smaller than that of the fixing roller, heat of the sheet passage unit, in the fixing roller, through which the recorded sheet continuously passes, is lost by the post cards, thereby decreasing the temperature. On the other hand, it is known that since the sheet non-passage unit is heated without loss of heat, the temperature therein increases. In this case, the temperature of the sheet non-passage unit is too much increased if the sheet passage unit is kept to have a proper fixing temperature. Consequently, when the next recorded sheet having a larger size passes, excessive melting is seen in the toner on the recorded sheet passing through the portion of the fixing roller having a higher temperature. If the toner is excessively melt, the viscosity of the toner is decreased and the toner is attached to the fixing roller without fixing to the recorded sheet. A phenomenon of so-called high-temperature offset occurs. On the other hand, if the sheet non-passage unit without the passage of the small-sized recorded sheet is to be kept to be a proper fixing temperature, the temperature of the sheet passage unit becomes too much low whereupon the toner cannot be melt and a phenomenon of a fixing defect is caused.
To prevent the above-mentioned inconvenience, the conventional apparatus controls (deceleration-controls) the number of passing sheets per time so as to be reduced in the halfway by prolonging interval between the conveyed recorded sheets while making a speed for image formation (process speed) constant at a timing of the passage of a predetermined number of sheets during the continuous copying operation or at a timing after a predetermined time. Thus, a difference of the surface temperature of the fixing roller in the longitudinal direction is controlled so as to be within an allowable range of a predetermined temperature.
However, in the conventional apparatus, if repeatedly executing an operation for allowing the recorded sheet having a small width such as a post card, to continuously passing and for ending the copying operation just before the deceleration control, the deceleration control cannot be performed in spite of increasing the difference of the surface temperature of the fixing roller in the longitudinal direction. As a consequence, there is a problem to cause the phenomenon of the high-temperature offset or defect of the fixing.
The present invention is devised in terms of the above circumstances and has its object to provide an image forming apparatus capable of reducing an image forming time, of preventing the surface temperature of the fixing roller from being out of the allowable predetermined temperature range, and of forming the image without the high-temperature offset and defect of the fixing, upon continuously image formation.
To accomplish the above-mentioned object, according to the present invention, there is provided an image forming apparatus including thermal fixing means having a fixing roller and heating means for heating the fixing roller, temperature detecting means for detecting a temperature of the fixing roller, and temperature control means for controlling power supplied to the heating means and controlling the temperature of the fixing roller based on a signal from the temperature detecting means, characterized by comprising: number of sheets storing means for storing the number of passing sheets upon previous continuous-image-formation; counter means for counting a passing time from the end time of the image formation; and control means for, upon this continuous-image-formation, based on the number of sheets stored by the number of sheets storing means and the time counted by the counter means, limiting the number of sheets image-formed at a first copying speed, at which the number of passing sheets per unit time is large, to a predetermined number of sheets, and for, when the number of sheets is over the limited number of sheets, changing the first copying speed to a second copying speed at which the number of passing sheets per unit time is small.